Compression issues

[Guest]

I'm very confused and could use some help.

I just rebuilt the engine on my 1982 GS1100. APE in California did the valves, cylinders, piston. I put in new rings. I assembled it according to the manual; it's not my first time. I adjusted the shims. Everything should be fine.

I tested compression and on 1, 2 & 4 it's around 90. On 3 its around 60. This seems low for what is essentially a new engine. I did a leak down on all pistons and 1,2 & 4 are fine. 3 is not.

Any ideas?

[azr]

Only guessing that you did the compression test with WOT

[BigVic54]

Only guessing that you did the compression test with WOT

What is WOT?

[azr]

Wide open throttle

[BigVic54]

Wide open throttle

WOT would lower the compression?

[azr]

As far as I know. The bikes all together? Carbs on that sort of thing

[azr]

Doing a compression test with the throttle closed will show inaccurate numbers much lower usually and they could easily be way off each other likely due to your butterflies not being synched

[Nessism]

Have you run the engine yet? Double checked the cam timing?

To do a proper compression test on a new engine you need to put some break in miles on it first. The engine should be hot and the throttle held wide open during the test too.

[BigVic54]

Thank you all for the suggestions.

[Nessism]

Look up the Moto Tune break in method. I don't think it's strictly necessary to follow their process to the letter, but a firm break in method is a very good idea.

[geol]

You did a leak down test but didn't say what was leaking on the cylinder with low compression. When you do a compression test, you should do it without the carbs installed OR hold the throttle open which is more difficult unless you have three hands. Readings won't be right otherwise... When you install new rings, a crosshatch hone is done so the rings can seat... the hone is essentially a relatively random scratching of the surface of the cylinder walls. There is no point in checking compression till you have the rings seat. To seat new rings, you should ride the bike and vary the engine speed as mich as possible for at least 500 miles or so. Break in applies more to the fact that in times past when engines were new, the parts were not made to the close tolerances that modern water cooled engines are today. The parts had to wear in together so they meshed together. You bike engine was broken in 35 years ago or so. You must need to seat in the rings. Perhaps the valves/valve seats will seat in a tad but if they lapped in the valves properly after the valve job, it shouldn't be necessary.